This application claims priority of Japanese application Nos. 2001-058397, 2001-058398 and 2001-058399. filed in the Japanese Patent Office on Mar. 2, 2001, and Korean application No. 2001-068302, filed with the Korean Industrial Patent Office on Nov. 2, 2001, the disclosures of which are incorporated herein by reference.
The present invention relates to a carbonaceous material and a lithium secondary battery comprising the same, and particularly to a carbonaceous material having a high charge-discharge capacity and improved cycle-life characteristics and a lithium secondary battery comprising the same.
As electronic products have become smaller, lighter in weight, and higher in quality and performance, demands for development of lithium secondary batteries exhibiting higher capacity have rapidly increased.
Although graphite, one candidate for a negative active material for a lithium secondary battery, has a theoretical capacity of 372 mAh/g, many efforts have been made to find an alternative material having a higher capacity.
Silicon or a compound thereof has already been proposed as an alternative to graphite since it is known to be capable of forming an alloy with lithium and providing a higher electro-capacity than that of graphite.
There are three recently suggested kinds of silicon material, namely: (1) a simple mixture material in which a silicon compound powder is added to graphite, (2) a graphite material in which a silicon compound particulate is chemically immobilized on the surface of the graphite by means of a silane coupling agent, and (3) a material in which a graphite-based carbon material and a metal material such as Si are bound and coated with an amorphous carbon material.
However, since the silicon compound is not firmly adhered to the graphite in the aforementioned simple mixture material (1), there is concern that the silicon compound may separate from the graphite due to the expansion and contraction of the graphite during the charge-discharge cycle. In addition, since the silicon compound has low electro-conductivity, it is not a sufficient negative active material because it degenerates the cycle characteristics of the lithium secondary battery.
Further, in the silicon compound particulate-immobilizing-graphite material (2), while adhesion of the silicon compound on the graphite is preserved in the early cycles of charge and discharge, rendering the silicon capable of acting as a negative active material, upon repeated charge and discharge cycles the silicon compound expands as a result of forming an alloy with the lithium so as to dissociate the bond and separate the silicon compound from the graphite. Also, the silicon compound is not a sufficient negative active material because it degenerates the cycle characteristics of the lithium secondary battery. Further, unless the silane-coupling agent is uniformly treated while preparing the negative electrode material, a negative electrode material having uniform quality cannot be obtained.
Lastly, the amorphous carbon material coated or bound on a graphite-based carbon material and a metal material such as Si (3) has the same problems as those of the (2) material. That is, upon repeated charge and discharge cycles, the bonding strength between the silicon compound and the amorphous carbon material becomes weaker so that the metal material is segregated from the graphite-based carbon material. Accordingly, the metal material is not a sufficient negative active material because it degenerates the cycle-life characteristics of the lithium secondary battery.
Japanese unexamined patent publication No. P5-74463 discloses a single-crystalline silicon being used as a negative active material, but the silicon has a low capacity at a low temperature. Further, the charge-discharge efficiency thereof at each cycle is relatively low, and the cycle characteristics degenerate in the case of adopting a Lewis acid base electrolyte such as LiBF4.
It is an object of the present invention to solve the above-mentioned problems, and to provide a carbonaceous material having a high C-rate capacity and improved cycle-life characteristics.
Further, it is another object of the present invention to provide a lithium secondary battery comprising the carbonaceous material.
In order to fulfill the objects, the present invention provides a carbonaceous material in which silicon and carbon are disposed in the vicinity of a graphite particle having a 002 plane interval d002 of less than 0.337 nm as measured by an X-ray wide angle diffraction method. In the carbonaceous material, complex particles having a particle size smaller than that of the graphite particle are also disposed and distributed. Both the graphite and the complex particles are coated with an amorphous carbon layer having a plane interval d002 of more than 0.37 nm, the amorphous carbon layer being a polymer layer. In the complex particles, a conductive carbon material is further disposed and distributed in the vicinity of the surface of the Si particulate, the Si 5 particulate being composed of crystalline silicon, and the Si particulate and the conductive carbon material are coated with a rigid carbon material layer. The Si particulate is characterized in that SiO2, SiC, and SiB4 phases are deposited on the crystalline Si phase.
The term xe2x80x9cvicinityxe2x80x9d herein is intended to express the positional relationship between the graphite particle and the complex particles, whereby the complex particles are located in contact with or adjacent to and marginally apart from the surface of the graphite particle.
The term xe2x80x9cvicinityxe2x80x9d is also intended to express the positional relationship between the Si particulate and the conductive carbon material, whereby the conductive carbon material is located in contact with or adjacent to and marginally apart from the surface of the Si particulate.
The term xe2x80x9cdispose and distributexe2x80x9d is intended to describe the state of distributing a plurality of complex particles without aggregating them with each other, and positioning the complex particles in contact with or adjacent to and marginally apart from the surface of the graphite particle.
The term xe2x80x9ccoatxe2x80x9d means the state of covering the subject particles to be coated, and binding the subject particles to be coated. In this case, the particles do not need to be in contact with each other.
Particularly, to coat the graphite particle and the complex particles with an amorphous carbon layer is to thoroughly cover both the graphite particle and the complex particles with the amorphous carbon layer rendering them bound together, or to locate the complex particles adjacent to the surface of the graphite particle within an amorphous carbon layer.
The amorphous carbon layer is obtained by heat-treating at least one polymer material selected from the group consisting of thermoplastic resins, thermosetting resins, vinyl-based resins, cellulose-based resins, phenol-based resins, coal-based pitch materials, petroleum-based pitch materials, and tar-based materials. The carbon layer is amorphous and is not relatively overly grown, and it preferably has a plane interval d002 of more than 0.37 nm.
Preferably, to coat the Si particulate and the conductive carbon material with a rigid carbon layer is to thoroughly cover both the Si particulate and the conductive carbon material with the rigid carbon layer rendering them bound together, and to locate the conductive carbon material adjacent to the surface of the Si particulate in the rigid carbon layer.
Further, xe2x80x9cdepositxe2x80x9d describes the state in which other deposited phases having compositions differing from that of the mother phase are incorporated in the mother phase. That is, SiO2, SiC, and/or SiB4 phases are deposited in the Si mother phase in an incorporated form, but it does not mean that the SiO2, SiC, and/or SiB4 phases are physically separated from each other.
The charge-discharge capacity is improved in the carbonaceous material of the present invention compared to the conventional case of a single graphite particle, since the Si particulate as well as the graphite can intercalate Li ions.
Also, the apparent conductivity of the Si particulate is improved by disposing and distributing a conductive carbon material in the vicinity of the surface of the Si particulate that has a high specific resistance.
The volumetric expansion and contraction of the Si particulate caused by reversibly intercalating the Li ions is mechanically suppressed by coating with the rigid carbon layer.
When the graphite and complex particles are covered with an amorphous carbon layer, the graphite particle does not directly contact the electrolyte, so electrolyte decomposition is inhibited. As a result, the complex particles are not separated from the graphite particle, and atomization of the Si particulate caused by volumetric expansion upon charging is inhibited.
The amount of the crystalline Si phase is relatively reduced since SiO2, SiC, and/or SiB4 phases are deposited therein so that the SiO2 phase is distorted, the crystalline Si phase become coarser, and excess occlusion of Li ions is suppressed. As a result, the expansion and contraction of the Si particulate caused by reversible intercalation of Li ions is appropriately suppressed. Since SiO2, SiC, and/or SiB4 phases do not react with Li, they do not have capacity, but they promote Li ion diffusion and they suppress the atomization caused by the volumetric expansion of the Si particulate.
The present invention can effectively provide a lithium secondary battery with the aforementioned features by means of SiO2, SiC, and/or SiB4 phases.
The carbonaceous material of the present invention increases the charge-discharge capacity and inhibits the degeneration of cycle-life characteristics by suppressing the atomization of the Si particulate caused by a variety of factors such as the volumetric expansion of the Si particulate, the separation of the complex particles from the graphite particle, and the volumetric expansion upon charging.
Silicon and carbon are mixed in a weight ratio of 0.1:99.9 to 50:50 in the carbonaceous materials of the present invention. When the weight ratio of silicone to carbon is less than 0.1:99.9, it is not preferred because the effect of silicon on improving the discharge characteristics is not obtained. Whereas when the weight ratio is in excess of 50:50, the atomization of the Si particulate resulting from the volumetric expansion is not sufficiently suppressed.
The Si:graphite:polymer weight ratio is preferably from 0.1:99.8:0.1 to 40:40:20.
The present invention effectively prevents degeneration of the cycle efficiency by blocking the separation of the complex particles from the graphite particle caused by the volumetric expansion of the Si particulate. The charge-discharge efficiency is improved since Li ions are rapidly diffused and reversibly intercalated in the battery that is charged with active material at a high density.
The carbonaceous material of the present invention is characterized as having a PSiO2/PSi ratio no less than 0.005 and no more than 0.1 and a PSiC/PSi ratio no less than 0.005 and no more than 0.1, wherein PSi is defined as the diffraction intensity of the plane (111) of the Si phase, PSiO2 is defined as the diffraction intensity of the plane (111) of the SiO2 phase, and PSiC is defined as the diffraction intensity of the plane (111) of the SiC phase, as measured by the X-ray wide angle diffraction method.
Where the SiB4 phase may also be deposited on the Si particulate, the PSiB/PSiO2 ratio is no less than 0.1 and no more than 5.0, and the PSiB/PSiC ratio is no less than 0.1 and no more than 5.0, wherein PSiB is defined as the diffraction intensity of the plane (104) of the SiB4 phase.
Since the carbonaceous material of the present invention is defined by the aforementioned ranges of diffraction intensity ratios, the amount of the Si phase is not dramatically reduced and the intercalation amount of Li is not degenerated. By optimizing the amounts of SiO2, SiC, and/or SiB4 phases with respect to the Si phase, the volumetric expansion and contraction of the Si particulate is suppressed.
The charge-discharge capacity characteristics are improved, and Si particulate is not separated from the graphite particle by suppressing the volumetric expansion of the Si particulate, preventing the atomization of the Si particulate caused by the volumetric expansion upon charging, and as a result, the cycle-life characteristics do not degenerate.
The carbonaceous material of the present invention is characterized in that the graphite particle has a particle size of 2 to 70 xcexcl, the complex particles have a particle size of no less than 50 nm and no more than 2 xcexcm, and the amorphous carbon layer has a thickness of no less than 50 nm and no more than 5 xcexcm.
When the graphite particle has a particle size of less than 2 xcexcm, the graphite particle would be smaller than the complex particles so that it would be difficult to uniformly adhere the complex particles onto the graphite particle, whereas when the graphite particle size is more than 70 xcexcm, its ability to attach to the collector degenerates, and concurrently the porosity of the electrode increases.
The complex particles have a particle size of no less than 50 nm and no more 5 than 2 xcexcm, and preferably no less than 50 nm and no more than 500 nm, in order to be uniformly distributed on the graphite particle since the complex particles should have a particle size of less than 2 xcexcm, which is the lower limit of the graphite particle. It is preferable for the complex particles to have a size of no more than 500 nm, since in this case the volumetric change of the complex particles caused by the expansion and contraction is reduced. It is not preferable for the complex particles to have a particle size of less than 50 nm, since in this case the crystal structure of the Si particulate included in the complex particles is seriously collapsed and the Li-intercalation amount decreases.
Further, when the amorphous carbon layer has a thickness of less than 50 nm, there is a concern that the graphite particle would not be completely covered with the amorphous carbon layer and that the complex particles may separate from the graphite particle. Whereas a thickness of more than 5 xcexcn is not preferable since the lithium ions cannot reach the graphite particle and the charge-discharge capacity decreases due to a decrease in the intercalation amount of Li ions.
Further, the carbonaceous material of the present invention is characterized in that the Si particulate has a particle size of no less than 10 nm and less than 2 xcexcm, the conductive carbon material has a specific resistance of no more than 10xe2x88x924 xcexa9xc2x7m, and the rigid carbon layer has a flexibility strength of no less than 500 kg/cm2 and a thickness of no less than 10 nm and no more than 1 xcexcm.
The reason for defining the Si particulate size at no less than 10 nm is to prevent the collapse of the crystalline Si structure and to increase the intercalation amount of Li ions. On the other hand, the reason for defining the particle size at less than 2 xcexcm is to have the complex particle size smaller than 2 xcexcm, which is the lower limit of the graphite particle size.
The reason for defining the specific resistance of the conductive carbon material at no more than 10xe2x88x924 xcexa9xc2x7m is to provide the Si particulate with sufficient conductivity.
The reason for defining the flexibility strength of the rigid carbon material at no less than 500 kg/cm2 is to mechanically suppress the expansion and contraction of the Si particulate and to decrease the volumetric change caused by reversible intercalation of the Li ions. Also, when the thickness of the rigid carbon layer is less than 10 nm, adhesive strength between the conductive carbon material and the Si particulate degenerates and the suppression effect of the volumetric expansion of the complex particles is eliminated. Whereas a thickness more than 1 xcexcm is not preferable since Li ions do not reach the Si particulate and the charge-discharge capacity degenerates.
Further, the carbonaceous material for the lithium secondary battery of the present invention is characterized in that the complex particles are present in the range of 1 to 25% by weight. Complex particles present in an amount less than 1% by weight is not preferable since the battery cannot obtain a charge-discharge capacity higher than the case of employing only the carbon material, by itself, as an active material. On the other hand, when the complex particles are present at more than 25% by weight, the relative content of the carbon material decreases so that the voltage at the early discharge state is increased to almost the Si reaction potential, and the cycle-life characteristics degenerate by frequent volumetric expansion and contraction of the Si particulate resulting from the narrower distance between the complex particles and the re-aggregation of the complex particles.
The present invention further provides a lithium secondary battery comprising the carbonaceous material.
The lithium secondary battery comprises, for example, a positive electrode, an electrolyte, and a negative electrode having the negative active material of the carbonaceous material. The lithium secondary battery according the present invention is in the form of, for example, a cylinder, a reticulum, a coin, or a sheet, and it may have other forms.
The present invention makes it possible to obtain a lithium secondary battery having a high energy density and superior cycle-life characteristics.
The present invention further provides a method for preparing a carbonaceous material comprising the steps of: calcining a Si particulate consisting of crystalline silicon in a carbon crucible at 1300 to 1400xc2x0 C. or calcining a Si particulate together with B2O3 powder in a carbon crucible at 1300 to 1400xc2x0 C. to deposit SiO2, SiC, and SiB4 phases in the crystalline Si phase; adding a conductive carbon material to the Si particulate; applying a polymer material coating solution to the Si particulate to provide a complex particle precursor; calcining the complex particle precursor to convert the polymer material coating solution into a rigid carbon layer to provide a complex particle; adding the complex particles to a graphite particle; applying a polymer material coating solution to the graphite particle to provide a carbonaceous material precursor; and calcining the carbonaceous material precursor to render the polymer material coating solution into an amorphous carbon layer to provide a carbonaceous material.